In the past, addiction was seen as a choice and under full control of the addicted person. The addict was seen as a transgressor engaged in morally wrong behaviour. The inability to control the use of substances was seen as the core of addiction. There is physical and psychological dependence. In the past, the measurement of addiction consisted of measuring the withdrawal. Addiction is not the same as a lack of control.

Addiction refers to a brain disorder characterised by compulsive behaviour. It is the inability to control despite awareness of the serious negative consequences on daily life. The attempts to reduce or quit substance use fail. A simplified view of addiction is that there is an imbalance between approach-oriented motivational system and a regulatory control system in the brain.

Repeated use of a substance leads to a sensitized and conditioned response to cues associated with the substance. In the case of addiction, the behavioural control is compromised. The reinforcement learning network (1), the executive control network (2) and salience network (3) are involved in the development of addictive behaviours.

The salience network is the hub for emotion regulation (1), salience attribution (2) and integration of affective information into decision making (3). The reinforcement learning network is the hub for associative learning from both positive and negative behavioural outcomes. It is involved in the development of automatic and habitual behaviour. The executive control network is the main substrate for relatively cold executive functions (e.g. processing events of low emotional salience). It is involved in executive control (e.g. attention, working memory, inhibition). 

The clinical symptoms of addiction include loss of control (1), social problems (2), pharmacological consequences (3) and high-risk use (4). The more symptoms a person has, the more severe the addiction is.

The environment plays an important role in the development of addiction as culture plays an important part in the development. Cannabis addiction appears to be real as 50% of the daily users develop an addiction. It is not clear whether behavioural addictions are also addictions. The harm of drug use is not equal to the severity of the addiction.

Adolescence marks the onset and escalation of drug use. At the age of 16, about 60% of the people use alcohol monthly. Binge drinking refers to drinking multiple glasses (e.g. >5) within 5 hours. This occurs relatively often among adolescents. It is associated with violence (1), risky behaviours (2), physical harm (3) and addiction (4). The proportion of substance-using friends is the best predictor as to whether a person will also use substances. Adolescents’ perception of peer substance use is related to their current substance use and substance use progressions.

There is an escalation of substance use in adolescence but also a de-escalation of substance use in late adolescence. This implies that adolescence is both a risk- and protective factor for the development of addiction.

Adolescent brain development is a key factor in the development of adolescent addiction. During adolescence, the grey matter thins and the white matter increases. This implies improved connectivity. The evolutionary older areas mature first (e.g. sensorimotor areas) and the areas involved in more complex functions mature later (e.g. cognitive control).

There are several reasons why adolescents are at risk for the development of addiction:

1. Hypersensitive salience network (i.e. hyperactive salience network)
   There is hypersensitivity in emotional and reward sensitivity. This stimulates exploration and independence but also stimulates risky behaviour. Striatal activity is related to binge drinking.
2. Hypersensitivity to social environment
   The sensitivity, complexity and reliance on peer relationship change rapidly during adolescence. This is paralleled with the improvements in introspection and the capacity to understand and care about others. Adolescents are more sensitive to peer pressure.
3. Facilitated brain plasticity and learning flexibility
   The brain is more flexible at a younger age. Addiction could be seen as an extreme case of neuroplasticity. In adolescence, the plasticity shifts towards brain areas involved in learning and complex cognitive functions. They also show better reinforcement learning and adolescents learn better from positive feedback. There might be enhanced hippocampal plasticity. This may, in turn, lead to a faster development of addictive behaviours.
4. Suboptimal control in tempting situations
   The different subprocesses regarding behavioural control mature at different ages. The strongest development takes place before adolescence. The ability to flexibly integrate contextual information and adapt decisions and behaviours accordingly develop more during later adolescence. This makes behavioural control in tempting situations more difficult in early adolescence.
5. Neurotoxicity during period of brain development
   Alcohol is extra dangerous for the developing brain. Substance use is associated with change in brain volume of areas important for behaviour control and it is related to poor cognitive performance.

Adolescents are less likely to take another individual’s perspective into account (1), are more sensitive to peer exclusion (2) and show more risk-taking in the presence of peers (3). The role of heritability in the development of addiction increases with age. The striatum is more active in cannabis users. Going against peer choices requires more effortful control for cannabis users. Furthermore, adolescents prefer short-term rewards over long-term rewards.

In adolescence, executive control functionality is highly sensitive to social and affective context. This supports the adaptive behavioural flexibility and social attunement to changing social environments. However, this imbalance may drive risky behaviour at first. Furthermore, in adolescence, there is a positive memory bias and this is associated with improved learning and enhanced functional connectivity between the hippocampus and striatum.

Brain plasticity refers to the dynamic biological capacity of the brain to change in response to the environment. This supports brain maturation as well as brain recovery after injury. It involves the modulation of neuronal connection through a complex interaction between genes and the environment, in which learning memory and the underlying genetic and cellular processes play an important role.

The earlier the onset of addiction, the worse the prognosis. This is associated with higher severity of the addiction and a higher relapse rate. During the transition from first-time use to substance dependence, the salience network becomes more attentive and responsive to substance use and substance use-related cues. Poor behavioural control over these motivations, resulting from decreased functioning of the frontoparietal executive network supports further escalation of substance use into addiction.

Adolescents who use alcohol for enhancement (i.e. improve social state) and coping purposes experience more alcohol-related problems than adolescents that drink for social motives.

Behavioural control refers to goal-directed regulation of behaviour. It includes control over emotions (1), impulses (2) and reflective mental processes (3). It supports flexible moment-to-moment adaptation of behaviour. It includes processes such as working memory, inhibition and attention.

Reactive control refers to control in response to things that are happening right now. Proactive control refers to behavioural control in anticipation of future events. Poor cognitive control predicts substance use and problems in both adolescence and adults. It is related to many disorders.

The evidence for facilitated brain plasticity and learning flexibility is mixed as well as for the neurotoxicity during a period of brain development. More evidence is needed. However, the other risk factors appear to be well-supported.

The prevalence of heavy substance use decreases at the end of adolescence. The same factors that put an adolescent at risk for heavy substance use are also resilience factors. The most important resilience factors are hypersensitivity to the social environment (1), facilitated brain plasticity and learning flexibility (2) and improving control in tempting situations (3).

There is a social devaluation of alcohol and drug use in later adolescence. This is associated with the transition to adult roles. This highly depends on culture and context. This social devaluation may be noticed early on by adolescents due to the hypersensitivity to the social environment, making hypersensitivity to the social environment a protective factor. The early reduction of brain plasticity (e.g. as a result of early onset of puberty) before substance use is socially devalued could hinder de-escalation of substance use.

In short, social devaluation with the optimization of behavioural control during a period where brain plasticity is high leads to adolescent resilience to substance use disorder.

Social attunement refers to the need to harmonize and attune one’s behaviour to that of peers. In early puberty, alcohol and cannabis may be seen as something negative. During adolescence, this changes to something positive and at late adolescence, it changes to something negative again. Resilience results from the development of proactive control, social attunement and the social devaluation of substance use. However, this has not fully been studied yet.

The social plasticity hypothesis refers to the idea that adolescent’s capacity to learn from and adapt to their constantly changing social environment is both a protective and a risk factor for the development of addiction.

Parents still influence adolescent behaviour and group therapy may be more effective when treating adolescents due to the influence of peers.